Copolymerizations of epichlorohydrin with glycidyl esters of ethylenically unsaturated acids or ethylenically unsaturated epoxides to produce solid elastomers which contain vinly unsaturation have been disclosed previously in U.S. Pat. No. 3,285,870 and 3,158,591. The prior art teaches preparation of trialkyl oxonium salts of HMF.sub.6 wherein M is an element selected from the group consisting of phosphorus, arsenic and antimony (U.S. Pat. No. 3,585,227) that are useful as catalysts for preparation of rubbery polyepihalohydrins (U.S. Pat. No. 3,850,857); and, when water or a glycol is employed as reactant, liquid hydroxyl-terminated epihalohydrin polymers (U.S. Pat. No. 3,850,856.